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Discussion papers
https://doi.org/10.5194/acp-2019-940
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-940
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Jan 2020

Submitted as: research article | 20 Jan 2020

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This preprint is currently under review for the journal ACP.

Vertical redistribution of moisture and aerosol in orographic mixed-phase clouds

Annette K. Miltenberger1,2, Paul R. Field1,3, and Adrian H. Hill3 Annette K. Miltenberger et al.
  • 1Institute of Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, United Kingdom
  • 2Institute for Atmospheric Physics, Johannes Gutenberg-University Mainz, Germany
  • 3MetOffice, Exeter, United Kingdom

Abstract. Orographic wave clouds offer a natural laboratory to investigate cloud microphysical processes and their representation in atmospheric models. Wave clouds impact the larger-scale flow by the vertical redistribution of moisture and aerosol. Here we use detailed cloud microphysical observations from the ICE-L campaign to evaluate the recently developed Cloud Aerosol Interacting Microphysics (CASIM) module in the Met Office Unified Model (UM) with a particular focus on different parameterisations for heterogeneous freezing. Modelled and observed thermodynamic and microphysical properties agree very well (deviation of air temperature < 1 K, specific humidity < 0.2 g kg-1, vertical velocity < 1 m s-1, cloud droplet number concentration < 40 cm-3, with the exception of an overestimated total condensate content and a too long sedimentation tail. The accurate reproduction of the environmental thermodynamic and dynamical wave structure enables the model to reproduce the right cloud in the right place and at the right time. All heterogeneous freezing parameterisations except Atkinson et al. (2013) perform reasonably well, with the best agreement in terms of the temperature dependency of ice crystal number concentrations for the parameterisations of DeMott et al. (2010) and Tobo et al. (2013). The novel capabilities of CASIM allowed testing of the impact of assuming different soluble fractions on dust particles on immersion freezing, but this is found to only have a minor impact on hydrometeor mass and number concentrations.

The simulations were further used to quantify the modification of moisture and aerosol profiles by the wave cloud. The changes in both variables are on order of 15 % of their upstream values, but the modifications have very different vertical structures for the two variables. Using a large number of idealised simulations we investigate how the induced changes depend on the wave period (100–1800 s), cloud-top temperature (−15 to −50 °C) and cloud thickness (1–5 km) and propose a conceptual model to describe these dependencies.

Annette K. Miltenberger et al.

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Status: open (until 16 Mar 2020)
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Annette K. Miltenberger et al.

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